Cardiovascular disease (CVD) in its various forms is the leading cause of death in the United States. Reverse cholesterol transport is a mechanism by which cholesterol present in atherosclerotic plaques within arterial walls is transported to the liver via high density lipoprotein particles for excretion in bile. Recent studies have suggested that human cholesteryl ester hydrolase (CEH), an enzyme that metabolizes cholesteryl esters, plays an important role in the regulation of reverse cholesterol transport. This enzyme is identical to the carboxylesterase CES1. Our long term goal is to understand the role that environmental toxicants such as agricultural chemicals play in human disease. Three commonly used organophosphate (OP) insecticides will be used in this proposed study. The hypothesis to be tested is that exposure to OP insecticides will inhibit the CEH/CES1-catalyzed metabolism of cholesteryl esters, which could therefore increase the risk of developing atherosclerosis. Three aims are proposed: (1) Determine the dose response curve for the oxons of chlorpyrifos, parathion, and methyl parathion that inhibit the cholesterol ester hydrolyzing activity of recombinant CEH/CES1 enzyme;(2) Determine the dose response curve for these same oxons with respect to inhibition of CEH/CES1 activity in a human monocyte/macrophage cell line (THP1);(3) Characterize the CEH/CES1 protein adducts formed after treatment of recombinant CEH/CES1 and THP1 cells with chlorpyrifos oxon. We will determine the potency of the active metabolites (oxons) of three environmentally relevant OP insecticides to inhibit CEH/CES1-catalyzed cholesteryl ester hydrolysis activity in a cell-free system and in cultured cells. Furthermore, we will identify the covalent adduct of the protein that inactivates enzyme function. Several environmental factors may increase the incidence of CVD in humans. The results from this study will provide preliminary insights into whether OP oxon metabolites can directly alter the structure-function of an enzyme involved in cholesterol metabolism, thus leading to an increased probability of a pathological outcome (i.e. atherosclerosis). These studies will determine if active metabolites (oxons) of three environmentally relevant organophosphate insecticides can interfere with cholesterol metabolism.